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MG MGB Technical - wierd electrics

ok, a bit of a wierd electrical problem, hope someone can help? About 2 months ago, i purchased (at a very resonable cost) a 1977 MGBGT Roadster. It had been standing for 5 years in a barn, yes they still exsist! only covered 2 miles since the last MOT since 2003! Sadly the rot had got a hold of the old girl around the floor, I have just finished welding new ones in.The engine runs well, and oil pressure is good, i have one niggly problem. First the pump would not run unless the side lights were on, so i put this at first down to a bad earth (earthing through the lights) but the ind light would glow, continuously? The wiring around the fuse box had the usual crud, and i cleaned it all off. Checking the alternator wiring i found that the ind wire had broken away. Now comes the wierd part. yes i could run a new wire straight to the dash, i don't want to do this yet but i can't find the other end of the broken wire? It does not go into the loom when it leaves the alternator, it sort of sits there. What i would like to know is does the feed wire come from the wiring block on the offside inner wing? or does it magically disappear somewhere?? looking through manuals doesn't really help
thanks in advance
john
john brandon

John. Please offer us a photo . MGB-GT is a hardtop. roadster is a soft top (hood).
Sandy
Sandy Sanders

Hi sandy, and thanks for the reply, sorry, i get confused easily :) its a hardtop :) (GT) no photos at the moment, as its still at work, i'm tring to make her look better :) I also have a 66 mustang, which is much easier to work on! :)

Cheers
john brandon

Normally when there is a bad ground supplying several things (like the fuel pump and the reversing lights) one thing at a time works well enough, it is when you turn two or more things on together that things go awry. Also the rear light clusters don't have a wired ground, they rely on their physical fixings to the wing. If the fuel pump now works then either it is coincidence and may stop again, or something is wired strangely.

The ignition warning light (red) is wired between the white/brown at the ignition relay contact as a white and the small spade on the alternator as a brown/yellow. A 77 had an ignition relay, but if it were a late 76 registered as a 77 and no relay the white side comes off the white at the ignition switch.

If the brown/yellow at the alternator has broken off then normally the warning light wouldn't glow at all. If is glowing all the time the ignition is on then the other end i.e. coming from the warning lamp must be earthing somewhere. Maybe the PO decided to earth this to get *some* indication from it, even though it is the wrong indication. The brown/yellow should disappear into the harness with the brown(s) and go back direct to the warning light, it doesn't involve the fusebox or relay, and that is what the manuals should show. Maybe you just need to unwrap it further, but don't reconnect it until you have discovered where the earth at the lamp end is coming from to light the lamp, and remove it, as just reconnecting the brown/yellow with it earthed will probably damage the alternator when the engine is running.

Also with the brown/yellow disconnected at the alternator you will probably find it doesn't start charging until the engine is revved to about 2k or 3k, instead of the normal 900rpm or so. But once it *does* start charging it should do so down to about 600rpm or so.
Paul Hunt


John, head over to:

http://www.advanceautowire.com/

and get a copy of the wiring schematics for the MGs.

You'll find it useful as you sort out electrics.

Pat
Pat Harrison

John - Follow Pat's advice on the wiring schematic for you car, take the file to some place that can open the file and print it full size and then laminate it. This will be your best troubleshooting help in that you can trace a wire with a grease pencil, then wipe it off when finished. Second go to Paul Hunt's web site and read everything about the electrics that are on it - wonderful information there. Finally, purchase a copy of Rick Astley's book MGB Electrics (someone please correct me if this is the wrong title) - an equally good source of information on the electrics for MGBs. Between those two sources, you should be able to dig all the problems out of your car. Cheers - Dave
David DuBois

Oops - Forgot to put the link to Paul's web site. It is: http://www.mgb-stuff.org.uk/ click on Spanners, then on electrics. Cheers - Dave
David DuBois

This thread was discussed between 07/06/2008 and 09/06/2008

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